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Originally Posted by hackish
The heads are about as good as you grandfather's 3.5hp briggs & straton lawn mower engine and they use huge quantities of fuel to keep the engine cool. The cylinder balance is horrible (they run per cylinder EGT) and they run 2 sparkplugs per cylinder.
They do not meet modern standards of efficiency or power output but nobody can afford to design,build and certify a modern replacement. I'm not sure about home-built or experimental, but the AC I'm familliar with have a TBO on the engines so every X hours you need to remove and rebuild them. Goodbye $75,000.
Since they have mixture adjustment and EGT/CHT probes there are a number of manufacturers who have approved an enleanment cycle during cruise to drop fuel consumption.
I think it would be nice if they could update the engines with a modern 4 valve OHC engine that is EFI and could run on non lead fuel. AFAIK you can't even buy unleaded at the pumps. 100LL is the best we can get.
Best thing I've seen so far is a turbo diesel conversion for some cessnas done down in texas I think. About $120k and it burns 1/3 less fuel and you fuel it on JET-A.
Sorry, but aircraft are not the pinnacle of efficiency.
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I've done some thinking on aero engines. Air-cooled direct drive aero engines represent a set of compromises that IMHO turned out better than we should expect. Light weight, compact, long service life, and very reliable. The all-up weight for an IO-520 is around 600 lbs. If you tried to build a "modern" water-cooled engine that is rated for 285 HP
continuous, all-day every day with a 1800 hr TBO, you'd need some kind of overbuilt, supercharged V-6 or V-8 that would weigh at least as much, if not more.
Fuel efficiency in air-cooled opposed direct drive engines can be quite good, too. (
GAMI has demonstrated BSFCs of less than .038 lbs/hp/hr, which approaches current diesels. This is with carefully calibrated mechanical fuel injection and magneto ignition.
I agree that modern fuel injection and ignition control could improve aero engines, and with redundant systems (like magneto ignition), reliability would be comparable. Cost would probably be similar.
I'm less convinced with things like overhead cams and multiple valves per cylinder. Direct-drive engines will never turn faster than about 2700 RPM due to prop limitations. At low RPM, two valves breathe better than four. You only need four when you want to rev fast, because the individual valves are much lighter and won't float. Same argument for pushrods -- they work fine at 2700 RPM, and a pushrod motor is much lighter and more compact than an overhead cam (see Chevrolet LS motor for support for this argument).
Adding a gearbox between the crankshaft and the propeller may be a better proposition today than it was years ago. They add to purchase cost, maintenance cost, and usually reduce TBO (see Continental TSIO-520 vs GTSIO-520, O-300 vs GO-300, etc), but with current manufacturing, maybe the cost difference can be reduced. I don't know enough about this one to comment. I do know that Bombardier was testing the waters for introducing a full-size engine (larger than their Rotax line) that would be a geared V-6, but they canceled it.
No-lead is coming. Engine manufacturers (and aftermarket folks like GAMI) are working on retrofittable electronic ignition that will allow almost all current engines (with the likely exception of the fire-breathing TIO-540s in your PA-31) to run on unleaded.
As for aero diesels, most of them produce less power and weigh much more than the O-320 or O-360 they're replacing. And then there's that $100K plus firewall-forward price. The
Deltahawk Diesel looks interesting, but it's not certified yet.
So to cut a rambling post short, don't be down on current aero engines. Yes, there's room for improvement, but they work amazingly well for 50-year-old tech!